This invention relates to detection circuits and more particularly to circuits used to detect low level signals in the presence of interfering continuous wave (CW) signals.
Amplitude Shift Key (ASK) or On-Off Key (OOK) modulation is commonly used in low cost RF systems for use in short range digital control or monitoring systems such as automotive Remote Keyless Entry, security sensor monitoring and garage door opening. In OOK modulation, a transmitter is switched on and off to create a data signal comprising xe2x80x9cmarkxe2x80x9d and xe2x80x9cspacexe2x80x9d periods. The received signal may have varying amplitudes during the xe2x80x9cmarkxe2x80x9d periods because of variation in distance between the transmitter and receiver. In ASK modulation, the data signal may be produced by switching a transmitter between high and low amplitude outputs in which energy is transmitted during xe2x80x9cspacexe2x80x9d periods in addition to during the xe2x80x9cmarkxe2x80x9d periods. In some forms of ASK modulation, intermediate amplitude levels may be used to provide a non-binary code. OOK modulation may be considered to be a form of ASK modulation.
The low cost systems mentioned above arc of an unlicensed category and commonly occupy narrow regions of the RF spectrum, eg. 433.92xc2x10.875 MHZ. Since these bands are not rigorously controlled however it is highly probable that there will be high level transmissions in the vicinity which can xe2x80x9cjamxe2x80x9d the wanted signal, interfering to such an extent that data is not recoverable at a receiver. Such interfering signals may be derived from many types of sources, for example, from amateur radio transmissions.
Receivers used in these low cost RF systems may be Tuned Radio Frequency (TRF), super-regenerative discrete designs and superhet integrated circuit designs for example. Transmissions to be detected are typically irregular and of short duration, say less than one second.
Suitable receivers often use highly selective RF filters prior to die Low Noise Amplifier (LNA) and at IF, although their bandwidths must allow for transmission frequency tolerances in excess of xc2x1100 kHz, with equally slack Voltage Controlled Oscillator (VCO) tolerances in the receiver for reasons of cost Data rates arc low, normally 0.4 to 5 kBits/s. Consequently, the signal bandwidth into a detector may be in the region of xc2x1300 kHz whilst the final data bandwidth is less than 5 kHz. Data filters may be employed to reduce noise bandwidth and increase sensitivity. However, the detector, being non-frequency selective, will detect on all signals, wanted and un-wanted, in the wide IF passband, giving high risk of jamming occurring.
A number of superhet designs for these applications use Logarithmic IF amplifiers and a Received Signal Strength Indicator (Log RSSI) as a detector. It is well known that these circuits are not capable of detecting signals in the presence of CW jammers of similar RF level. However tie benefit of Log RSSI detectors is their instant response time over a very wide range of signal level, ideal for OOK signals, compared to using a linear IF amplifier with automatic gain control.
In band continuous interfering IF signals reduce the sensitivity of the RSSI detector to the wanted signal, as the output is a log function of the input RF level during the xe2x80x98markxe2x80x99 and xe2x80x98spacexe2x80x99 periods. Also during the xe2x80x98markxe2x80x99 period such interfering IF signals produce a signal modulated at the beat frequency (fwxe2x88x92fj), where fw is the wanted IF signal and fj is the jamming signal. This reduces the mean level of the wanted signal after it is filtered by a data filter to the extent that, for a jamming signal at the same level or greater than the wanted signal, the output data signal is zero. FIG. 1 shows part of the beat frequency envelope during the xe2x80x98markxe2x80x99 period of the wanted signal when the interferer is 6 dB higher in power. FIG. 2 is an explanatory diagram of the detected signal at the RSSI output of the log amplifier. Mean signal levels during xe2x80x9cmarkxe2x80x9d and xe2x80x9cspacexe2x80x9d periods are shown with the levels for xe2x80x9cmarkxe2x80x9d and xe2x80x9cspacexe2x80x9d periods being inverted when compared to the wanted signal without interferer. The modulation component V is impressed on the wanted signal and when data is present (not shown in the diagram) the xe2x80x9cmarkxe2x80x9d and xe2x80x9cspacexe2x80x9d amplitudes will vary with it
The RSSI output is put through a data filter to reduce the noise bandwidth. However the CW jamming signal produces a beat frequency such that there is no longer a low frequency data component present, as shown in FIG. 3.
Typically systems using Log RSSI detectors arc only capable of detecting wanted signals with signal level ratios of:             P      jam              P      wanted        ≤            -      3        ⁢          xe2x80x83        ⁢    dB  
According to die invention there is provided a detection circuit for a modulated signal having mark and space intervals comprising de restoration means for de restoring a received modulated signal such that signal peaks corresponding to mark intervals to give a dc restored signal and reject interfering signals. The signal applied to the de restoration means may undergo some processing prior to being applied to the de restoration means. The received modulated signal is ac coupled to the dc restoration means.
This invention considerably reduces the effect of beat frequencies due to jamming signals in the IF, enabling the wanted signal to be detected in the presence of a jamming signal level ratio of:             P      jam              P      wanted        ≤            +      20        ⁢          xe2x80x83        ⁢    dB  
By using the invention, it is possible to recover a low level wanted ASK signal from CW interferer which would otherwise prevent the wanted signal from being recovered. An anti-jam ASK detection circuit in accordance with the invention may enable ASK and OOK signals to be detected in the presence of +20 dB CW jamming signals relative to the wanted xe2x80x9cmarkxe2x80x9d level. This gives an overall improvement of +23 dB over the RSSI detector alone.
The detection circuit may also be used as the detector directly on the signal output of a linearly automatic gain controlled IF amplifier where greater performance improvement may be realised.
According to a first feature of the invention, a receiver includes a detection circuit in accordance with the invention.
According to a second feature of die invention, a system includes a transmitter for transmitting a modulated signal having mark and space intervals and a receiver incorporating a detection circuit in accordance with the invention.